In recent years, water-resistant photographic printing paper supports which consist of a base paper covered on both sides with a coating of a polyolefin such as polyethylene, have been frequently used for the rapid development of photographic printing paper.
Such photographic printing paper supports consisting of a base paper covered on both sides with a polyolefin coating include ones which have glossy surfaces, ones having matte surfaces, and ones whose surfaces have patterns such as a silk texture. Most advantageously employed of those are supports having smooth and glossy surfaces with no patterns, and particularly preferred are ones in which both surfaces have very few fine irregularities to give a mirror-like smoothness.
In obtaining such supports having a good surface smoothness, various papers have been proposed as a base for the supports. For example, JP-A-60-67940 discloses a base paper prepared by use of a pulp in which the content of voids not larger than 0.4 .mu.m in diameter is 0.04 ml/g or more (The term "JP-A" as used herein means an "unexamined published Japanese patent application"); JP-A-60-69649 discloses a base paper prepared by use of a wood pulp having an average fiber length of 0.4 to 0.9 mm, an average fiber width of 13.5 .mu.m or more, and an average fiber thickness of 4 .mu.m or less; JP-A-61-275752 discloses a base paper prepared by use of a fiber mixture composed of a natural pulp and 5 to 60% hydrophobic fibers; and JP-A-61-284762 discloses a base paper prepared by a method in which when a wet web is obtained from a pulp slurry by means of a twin-wire paper machine, dehydration is conducted under specific conditions. Furthermore, there is also employed a method in which a base paper is subjected to calender treatment between a metal roll and another metal roll at an increased machine calender pressure, in order to densify the base paper which is used in a photographic printing paper support. On the other hand, for the coating of base papers with a polyolefin such as polyethylene, there generally is employed an extrusion coating process, in which a molten polyolefin is extruded at a high temperature over the surface of the base paper thereby to apply a coating. In order to improve the smoothness of the photographic printing paper supports, the above extrusion coating is performed in such an improved manner that the thickness of the polyolefin coating is increased or the pressure applied when the polyolefin coating is formed is increased.
However, the above improvements in the polyolefin coating process are not very effective and are also disadvantageous in regard to cost. Moreover, the above-described method to densify the base paper by means of machine calender treatment is also disadvantageous in that appearance defects such as blacking and cockles are apt to result therefrom. Accordingly, the above known methods cannot produce photographic printing paper supports having satisfactorily smooth surfaces, because of the presence of irregularities on the base paper. This applies of course to a base paper having irregularities on its front side, and also applies to a base paper having irregularities on its back side with the front side being smooth; in the latter case, the irregularities on the back side affect the polyolefin covering being extrusion-coated on the front side, depending upon the degree of the back side's irregularities.